1. Field of the Invention
The present invention relates generally to printing controls and, in particular, to a printing apparatus and method for suppressing emission of excess dilution liquid when a mixing concentration between an ink and a dilution liquid on a recording paper sheet is varied for expressing a half-tone print.
2. Description of the Related Art
A related printing apparatus, a so-called on-demand type ink jet printer, emits an ink droplet responsive to a recording signal for recording signals on a recording medium, such as paper or film. This type of printing apparatus has recently come into widespread use since it can be reduced in size and cost.
In addition, document preparation employing computers (i.e., desk-top publishing) has increased dramatically in recent years, particularly in office environments. Recently, the demand has increased for devices that can output not only a letter or a figure, but also a natural picture, such as a photograph, along with a letter or figure. For printing a natural picture of high picture quality, regeneration of a printing half-tone is crucial.
An ink jet print head used in an on-demand type ink jet printer, referred to hereinafter as a print head, employs a piezoelectric vibrator, such as a piezo-element, or a heating element for emitting an ink droplet. A print head utilizing displacement of a piezoelectric vibrator, such as a piezo-element, emits an ink droplet as a medium for deposition by deforming the piezo-element for pressurizing the ink.
A print head that utilizes displacement of a piezoelectric vibrator can use the so-called multiple concentration ink mixing type ink dot concentration modulation method (in-ink-dot concentration modulation method) for producing a desired half-tone print.
This in-ink-dot concentration modulation method is a technique for metering the concentration of the emitted ink liquid droplet. Specifically, the voltage level applied to the piezoelectric vibrator is varied responsive to the variation in the impressed voltage level for varying the volume of the ink liquid droplet to be mixed responsive to the displacement of the piezoelectric vibrator for producing the desired half-tone print.
For the ink liquid droplet, a mixed liquid of an ink of high concentration and a diluted ink liquid is used. For mixing the ink liquid droplets, a method is provided for mixing the ink of high concentration and the diluted ink liquid, which are metered in an ink head responsive to the displacement of a piezoelectric vibrator. The mixing occurs prior to emitting the ink liquid droplet via a nozzle. Alternatively, a method is provided for separately emitting the ink of high concentration and the diluted ink liquid, which are metered in an ink head responsive to the displacement of the piezoelectric vibrator. In this method, the mixing occurs at the time of emitting the ink liquid droplet, with the ink and the diluted ink liquid being mixed together during flight or upon deposition on the recording paper sheet for forming a single ink dot.
For regenerating the half-tone in the above-described on-demand type ink jet printer, there is a method of forming a pixel using a matrix of 4xc3x974 dots, and expressing the gradation using a so-called dither method on the matrix basis.
The amount of ink of various colors is metered on the basis of the picture printing data signals produced using this dither method. The ink liquid droplets comprising a mixed liquid of the respective ink liquids and the diluted ink liquids are emitted from the print heads associated with the inks of various colors.
The picture printing data signals are 8-bit signals of, for example, 000 to 255, in which 000 and 255 correspond to the stop of the ink emission and the maximum amount of the emitted ink (i.e., the highest ink concentration), respectively.
The ink used is in the form of a combination of C (cyan), M (magenta), and Y (yellow) or C (cyan), M (magenta), Y (yellow), and K (black).
With the above-described on-demand type ink jet printer, if the picture printing data signal for a print head is 000 for a given print head and is not 000 for another print head, ink emission from the print head with the picture printing data signal of 000 is stopped, while the diluted ink liquid from the same print head is continued.
At this time, the total amount of the ink dilution liquids emitted from the respective print heads becomes superfluous, such that the ink liquid droplet can hardly be dried, and bleeding or flow of the ink liquid droplet is liable to be produced. The result is a suppressed picture printing speed and waste of the dilution liquid.
Thus, it has been proposed to stop the emission of the ink and the dilution liquid in all of the print heads, if the picture printing signal for a given print head is 000, irrespective of whether or not the picture printing signal for another print head is 000.
However, since no ink liquid droplet is emitted to the respective pixels of a totally white area on a recording medium, the picture printing concentration in the vicinity of the area is changed acutely to produce tone jump and a so-called pseudo-contour in the vicinity of the area. Although the tone jump or the pseudo-contour raises no problem as long as they are not outstanding, they can affect the picture quality significantly.
In view of the above-described problems in the related art, it is an object of the present invention to provide a printing apparatus in which emission of a superfluous dilution liquid is suppressed for suppressing tone jump or generation of a pseudo-contour for improving the picture printing speed and realizing high quality picture printing.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In order to solve the above-described problems, a printing apparatus according to the present invention comprises: a plurality of print heads for printing a plurality of respective colors based on a printing data, the print heads each having a means for discharging an ink and a dilution liquid at varying concentrations based on the printing data; and a stop means for stopping a discharge of the dilution liquid from a first one of the print heads when the printing data corresponding to a dot to be printed by the first print head is below a preset threshold value, while permitting a discharge of dilution liquid from a second one of the print heads whose printing data corresponding to the dot is above the preset threshold value.
According to a second aspect of the present invention, a printing apparatus is provided which comprises: a plurality of print heads for receiving a printing data and discharging an ink and a dilution liquid; and a stop means for detecting a printing data to be inputted into the plurality of print heads for a dot to be printed, and for stopping a discharge of the ink and dilution liquid from at least one print head whose printing data corresponding to the dot to be printed is below a predetermined threshold value, while permitting a discharge of the ink and dilution liquid from another print head whose printing data corresponding to the dot to be printed is above the predetermined threshold value.
According to a third aspect of the present invention, a method of driving a printing apparatus having a plurality of print heads for printing a plurality of respective colors based on a printing data is provided. The print heads each have a means for discharging an ink and a dilution liquid at varying concentrations based on the printing data. The method is characterized by the steps of: receiving printing data which determines an ink concentration for a dot to be printed; comparing the printing data to a preset threshold value; stopping a discharge of dilution liquid from a first one of the print heads whose printing data corresponding to the dot to be printed is at or below the preset threshold value; and discharging dilution liquid from a second one of the print heads whose printing data corresponding to the dot to be printed is greater than the preset threshold value.
According to a fourth aspect of the present invention, a method of driving a printing apparatus having a plurality of print heads for mixing an ink and a dilution liquid and for varying an ink concentration within a dot to be printed on a printing medium is provided. The method is characterized by the steps of: receiving a printing data which determines an ink concentration; comparing the printing data to a predetermined threshold value; stopping a discharge of the ink and dilution liquid from at least one of the print heads whose printing data corresponding to a dot to be printed is at or below the predetermined threshold value; and discharging the ink and dilution liquid from at least another one of the print heads whose printing data corresponding to the tot to be printed is greater than the predetermined threshold value.
As described above, the present invention uses, as the medium for deposition, a liquid mixture comprising an ink metered responsive to the picture printing data signal and a dilution liquid for the ink.
The printing apparatus of the present invention is made up of a plurality of print heads in association with the inks of various colors for emitting the associated media for deposition. The printing apparatus is characterized by a stop means for stopping the emission of the dilution liquid when the printing picture signal falls below a preset threshold value. Specifically, the stop means operates so that, if the picture printing data signal sets the amount of ink emission in one or more of the print heads to a value not larger than a preset threshold value while setting the amount of ink emission in another print head to a value larger than the threshold value, the emission of the dilution liquid is stopped along with the emission of ink in the one or more print heads.
If the amount of ink emission in a given print head is set to a value smaller than the threshold value, the ink concentration is extremely low such that, on visual inspection, no definite picture printing concentration is achieved. Therefore, if a dilution liquid is emitted from the print head at this time, the dilution liquid would be superfluous.
Thus, with the printing apparatus of the present invention, if the amount of ink emission from one or more print heads is set to a value smaller than the threshold value, while the amount of ink emission in another print head is set to a value larger than the threshold value, emission of the superfluous dilution liquid is suppressed in the one or more print heads. Therefore, it becomes possible to prevent bleeding or flow of the medium for deposition on the recording medium, as well as to accelerate drying of the medium for deposition.
In the present invention, it is desirable to prepare the printing picture data signal using a multigradation error diffusion method.